1. Field of the Invention
The present invention generally relates to data transmission apparatuses. More particularly, the present invention relates to a data transmission apparatus which inhibits data transmission in case of overflow of data when data is transmitted autonomously and asynchronously from the first unit to the second unit.
2. Description of the Background Art
A data transmission apparatus which autonomously and asynchronously transmits data from the first data path to the second data path is disclosed, for example, in U.S. Pat. No. 4,985,890. When a transmission permitting signal is transmitted from the second data path to the first data path, data is transmitted from the first data path to the second data path, while the first data path transmits the transmission prohibiting signal to a preceding stage until the data transmission from the first data path to the second data path is completed. After the data transmission from the first data path to the second data path, the first data path provides the transmission permitting signal to the preceding stage to hold new data transmitted from the preceding stage.
In the data transmission device described above, at the time of data transmission from the first data transmission path to the second data transmission path, if a propagation delay time in the second data transmission path is longer than that in the first data transmission path, continuous increase in the .number of data pieces accepted in the second data transmission path in proportion to an length of running time leads to the excess number of data pieces, resulting in decrease in processing speed and overflow depending on the contents of processing.